1. Field of the Invention
This invention relates to the production of high-pyruvate xanthan gum by culturing Xanthomonas campestris on a novel synthetic growth medium.
2. Description of the Prior Art
Xanthan gum is a heteropolysacchride produced as a fermentation product by X. campestris. Its structure consists of .beta.-(1.fwdarw.4)-linked D-glucosyl backbone chain with alternate residues having appended thereon a three-unit side chain of D-mannose and D-glucuronic acid in the molar ratio of 2:1. Half of the side chain D-mannosyl residues are attached directly to the main chain through .alpha.-(1.fwdarw.3) linkages. The remaining D-mannosyl residues occur as nonreducing end groups. Approximately half of these D-mannosyl groups carry pyruvic acid as the di-O-4,6-ketal.
Although X. campestris is not difficult to cultivate on standard laboratory media, certain strain variations have been observed, both in continuous and batch-type fermentations, which affect the quality and yield of the gum. Variation was first associated with the formation of large (L) and small (S) colony types [M. C. Camus et al., Can. J. Microbiol. 22: 943 (1976)], herein incorporated by reference. The L-type produced high yields of the heteropolysacchride with apparently normal rheological characteristics, while the S-type gave low yields with undesirable properties. The only detectable structural difference was a significantly lower pyruvate concentration in the S-type polysaccharide. It was subsequently shown in greater detail (P. A. Sandford et al., Polysaccharides of Industrial Importance Symposium, 172nd Meeting, American Chemical Society, August 29-September 3, 1976) that viscosities of dilute (0.5% or less), aqueous, xanthan solutions are directly related to the pyruvic acid content of the gum. In fact, it appears that the pyruvic acid content is the best indicator of polysaccharide quality.
Xanthan gum is produced in both the United States and Europe, and has numerous applications in food and nonfood industries. Many of these applications require a quality product having a pyruvic acid content above about 3.3% by weight and being substantially free of insoluble material and/or color. Most product methods, such as those disclosed in U.S. Pat. Nos. 3,054,689, 3,594,280, and 3,671,398 use an organic nitrogen source (e.g., dried distillers' solubles) in a carbohydrate-containing nutrient medium. Thes organic materials are disadvantageous in that they are not always readily available, they sometimes fail to stimulate production of high-pyruvate polysacchrides, and they reduce the gum quality by virtue of their residual insolubles and dark coloration. The requisite purification procedures for obtaining an acceptable product from these gums are quite elaborate and costly.
An attempt to overcome the problem of the insolubles is taught in U.S. Pat. No. 3,391,060, wherein inorganic ammonium nitrate is substituted for the organic nitrogen nutrient. Although this approach was somewhat successful, both polysaccharide yields and pyruvate content were undesirably low. Another inorganic nitrogen source, diammonium phosphate, has been utilized in U.S. Pat. No. 3,438,865 as a nutrient for Escherichia coli in the production of lipopolysaccharides. However, its usefulness in culturing X. campestris has not been previously taught.